Journal of the Marine Biological Association of the United Kingdom, 2013, 93(1), 135–142. # Marine Biological Association of the United Kingdom, 2012 doi:10.1017/S0025315412000938 Population abundance and body size of sinicus in marginal habitats in the coastal seas of south-eastern Hong Kong g.-t. zhang1 and c.k. wong2 1Jiaozhou Bay Marine Ecosystem Research Station, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China, 2School of Life Sciences, The Chinese University of Hong Kong, Shatin, Hong Kong, China

The species range of Calanus sinicus along the Chinese coast extends from the Bohai Sea and the Yellow Sea in the north to the northern part of the South China Sea in the south. The subtropical seas along the southern coast of China mark the southern edge of the range of C. sinicus. In coastal seas off eastern Hong Kong, C. sinicus appears first in December, but densities com- parable to those in the Yellow Sea and the East China Sea are reached only in January and February when temperature is ,158C. Density decreases in March as temperature increases. No individuals remain after May when temperature is .258C. The average prosome length of females collected in February is comparable to that of females from the Yellow Sea, but females collected after mid-March are smaller than the smallest females from the Yellow Sea. Reproduction occurred mainly between January and March. Rapid decline of the population in April and the absence of a summer population suggest that the local population is derived from individuals advected from the north by ocean currents. Eggs produced locally probably did not hatch or develop into adults.

Keywords: Calanus sinicus, population abundance, body size, subtropical coastal waters

Submitted 22 May 2012; accepted 5 June 2012; first published online 4 September 2012

INTRODUCTION season from April to September. Oceanic conditions, charac- terized by high salinities, prevail on the eastern side of Hong Calanus sinicus is a calanoid with wide distribution Kong. The eastern side of Hong Kong consists of two semi- in continental shelf waters of the north-west Pacific Ocean enclosed bays in the north and open sea area in the south (Hulsemann, 1994). Along the Chinese coast, the range of (Figure 1). Tolo Harbour, a landlocked bay opens into Mirs C. sinicus extends from the Bohai Sea and the Yellow Sea in Bay via a narrow tidal channel. Mean water depth in Tolo the north to the northern part of the South China Sea in the Harbour varies from 10 m in the inner shallow basin to south (Chen, 1992). In the temperate waters of the Bohai about 20 m in the channel. Because of the semi-enclosed topo- Sea, the Yellow Sea and the East China Sea, C. sinicus graphy, Tolo Harbour is characterized by low current velocity occurs throughout the year and is a dominant member of and is poorly flushed (Oakley & Cripps, 1972). Bordered by the mesozooplankton in terms of numerical abundance and Hong Kong on the west and mainland China on the north ecological importance (Chen, 1992; Wang et al., 2003). In sub- and east, Mirs Bay has an average water depth of about tropical seas along the coast of southern China, C. sinicus 25 m and is fully exposed to water currents from the South occurs seasonally from late autumn to early spring (Chen, China Sea. 1992; Hwang & Wong, 2005). While C. sinicus is considered Results of a previous study revealed that the occurrence of one of the key species of planktonic in the coastal Calanus sinicus in coastal oceans around Hong Kong and waters of China and Japan (Chen, 1964), most previous Taiwan is limited by temperature to winter and spring studies on the seasonal dynamics and life history of C. (Hwang & Wong, 2005). It was hypothesized that C. sinicus sinicus have focused on populations in temperate oceans is carried into the subtropical oceans in the southern part of (Uye, 2000; Zhang et al., 2005; Xu & Chen, 2007; Zhang the species range every winter from population centres in et al., 2007). Far less information is available on populations the north by the China Coastal Current when the north- in the subtropical waters near the southern edge of the eastern monsoon is blowing (Chen, 1992). Understanding species range. the life history of C. sinicus in marginal habitats near the Hong Kong is located in the southern part of the Chinese southern edge of its geographical range is important to coast. A plume of estuarine water from the Zhujiang River researchers of the ecosystem dynamics of the South China covers the western side of Hong Kong during the rainy Sea. In this study, seasonal variations in population structure and body size of C. sinicus are investigated along a transect extending from the inner part of Tolo Harbour to the outer

Corresponding author: part of Mirs Bay. The objectives are to: (i) investigate the C.K. Wong population structure of C. sinicus in subtropical coastal Email: [email protected] oceans; (ii) identify factors that influence the seasonal and

135 136 g.-t. zhang and c.k. wong

Fig. 1. Map of Hong Kong showing the locations of Tolo Harbour, Mirs Bay and the six sampling stations.

spatial distribution patterns of C. sinicus; and (iii) provide pre- by hauling a conical plankton net (0.5 m mouth diameter; liminary information on the recruitment mechanisms of C. 125 mm mesh) vertically from 2 m above bottom to the sinicus in marginal habitats in the northern part of the surface. All zooplankton samples were immediately preserved South China Sea. in 4% formalin seawater. Water temperature and salinity were measured at the surface (0.5 m) using a Hydrolab H20 water quality monitoring system (Austin, USA). Chlorophyll-a con- MATERIALS AND METHODS centration in seawater samples collected at the surface (0.5 m) with a Van Dorn type bottle was measured fluorometrically Zooplankton was collected approximately semi-monthly from using a Turner Designs 10-AU-000 Fluorometer (Sunnyvale, September 2003 to July 2004 at 6 sampling stations. Two USA) (Strickland & Parsons, 1968). stations, S1 (22826′725′′N, 114814′566′′E) and S2 In the laboratory, C. sinicus in different developmental stages (22828′539′′N, 114818′034′′E) were located inside Tolo (eggs, nauplii (NI–NVI), copepodites (CI–CV), adult males Harbour. Two stations, S3 (22829′468′′N, 114821′588′′E) and and adult females) were identified and sorted under a stereomi- S4 (22826′670′′N, 114826′920′′E) were located in Mirs croscope according to the descriptions of Li & Fang (1983). Bay. Two stations, S5 (22817′560′′N, 114826′920′′E) and S6 Densities were determined by counting 25–100% of each (22813′000′′N, 114826′920′′E) were located in the coastal sea sample under a stereomicroscope. Prosome lengths, from the of south-eastern Hong Kong outside Mirs Bay (Figure 1). anterior end of the cephalosome to the posterior lateral edge Water depths ranged from 15 m at S1, the innermost of the last metasome segment, of CVs, adult males and adult station, to 30 m at S5 and S6, the two outermost stations. females were measured. At least 10 individuals of each develop- At each station, duplicate zooplankton samples were taken mental stage were measured for each sample.

RESULTS

Environmental conditions Similar seasonal trends in temperature and salinity were observed at all sampling stations (Figure 2). Surface water temperature ranged from 158C in late January and early February to .258C in May. Salinity ranged from a maximum of 37 at S2 in February to a minimum of 31 at S1 and S3 in early May (Figure 2). Salinity decreased markedly after March, especially in Tolo Harbour, with the approach of the rainy season. Chlorophyll-a concentration was much higher in Tolo Harbour than in Mirs Bay (Figure 3). In Tolo Harbour, values spanned widely from 2.67 to 20.30 mgl21 at S1 and m 21 Fig. 2. Temporal variations in water temperature and salinity in the study area from 0.84 to 8.70 gl at S2. In comparison, values recorded during the period when Calanus sinicus was present. Values represent average at the four stations outside Tolo Harbour varied from 0.17 to 21 of six sampling stations. Error bars denote the range. 4.36 mgl . calanus sinicus in the coastal seas of south-eastern hong kong 137

in April and May. Eggs of C. sinicus appeared first in samples collected in December but did not become abundant until late January. The maximum average density of 485 ind. m23 was recorded in early February. No eggs of C. sinicus were found in samples collected in May. Calanus sinicus exhibited spatial variations in abundance throughout the study period (Figure 4). Adults, copepodites, nauplii and eggs appeared at S5 and S6, the outermost stations in December, but did not reach Tolo Harbour until January. Adults and copepodites appeared at S5 and S6 in December, reached S3 and S4 in Mirs Bay in early January, and were found in Tolo Harbour at the end of January. Throughout January and February, the density of adults was much lower at S1 and S2 than at the four outer stations. A similar trend of shoreward decrease was observed in the density of nauplii and copepodites in January. Samples collected in Tolo Harbour in early January contained no nauplii. A few eggs were found in S2 in early January when the average density of eggs in the entire study area was still ,50 m23, but samples collected in Tolo Harbour in late January did not contain eggs even though egg density at S6, the outermost station, had reached a maximum of .1400 m23. While the density of adults was always highest in the open waters outside Tolo Harbour, samples collected in March showed that eggs, nauplii and copepodites were often abundant at S2 in the outer part of Tolo Harbour and S3 in the inner part of Mirs Bay than at the other stations. Eggs and nauplii disappeared from Tolo Harbour before adults and copepo- dites. No eggs and nauplii were found in Tolo Harbour after March. In contrast, adults and copepodites remained at S1 until April and at S2 until early May.

Sex-ratio and female fecundity The ratio of female:male was highest in December when the density of adults in the study area was still very low (Figure 5). Sex-ratio decreased after December. Females were outnumbered by males only in samples taken in early Fig. 3. Temporal variations in mean chlorophyll-a concentrations (error bars: March. The number of eggs per female increased from ,15 standard deviation) at six sampling stations during the period when Calanus in December to a peak of 111 in early March. The number sinicus was present. of eggs per female decreased markedly after March and no ovigerous females were found in samples taken in May. Population abundance and structure Prosome length Calanus sinicus was found only in samples collected between late December and early May (Figure 4). The highest densities Temporal and spatial changes in the prosome lengths of adults were recorded in January and February. Averaged across and copepodites of C. sinicus are shown in Figure 6. The all sampling stations, the density of adults increased from prosome length of females, males and CV varied significantly 1.1 ind. m23 in late December to 78.8 ind. m23 in early among stations and with time (2-way analysis of variance, P , February, then decreased to 13.2 ind. m23 in early March 0.001). The overall pattern was that individuals from January and ,1.6 ind. m23 in April. No adults were found in and February were bigger than individuals from March, April samples taken in May. Copepodites also appeared first in and May (Tukey’s multiple comparison tests, P , 0.05), and December and remained relatively abundant until early individuals from S3, S4, S5 and S6 were bigger than individ- March. The maximum average density of 402 ind. m23 was uals from S1 and S2 (Tukey’s multiple comparison tests, recorded in late January. Average density dropped abruptly P , 0.05). Averaged across all stations, the prosome length from 384 ind. m23 in early March to 38.3 ind. m23 at the of female increased from 2.29 mm in December to 2.34 mm end of March. Samples collected in April and May con- in early February, and then decreased gradually to 1.98 mm tained ,30 ind. m23. Only a few nauplii of C. sinicus were in early May (Figure 7). Similarly, the average prosome found in samples collected in December. Average density length of male increased from 2.03 mm in December to increased markedly from ,30 ind. m23 in early January to 2.18 mm at the end of January, and then decreased to .1100 ind. m23 at the end of January, and then decreased 1.86 mm in May (Figure 7). Only one CV individual was more or less continuously to ,10 ind. m23 in samples taken found in samples taken in December. The average prosome 138 g.-t. zhang and c.k. wong

Fig. 4. Temporal variations in the density of Calanus sinicus of different developmental stages at six sampling stations. Solid lines connect average densities of six sampling stations.

length of CV was 1.87 mm in early January, 1.99 mm at the increases rapidly in January. By the end of January, individuals end of January, and 1.67 mm in May (Figure 7). of all developmental stages can be found in the innermost station in the shallow waters of the semi-enclosed Tolo Harbour. The highest density is recorded in late January and early February, when average water temperature has DISCUSSION dropped to ,158C. Densities recorded in January and February are comparable to those encountered in the popu- Population dynamics lation centres in the Yellow Sea (Liu et al., 2003; Wang et al., 2003). Overall, observations from this study and an Calanus sinicus appears in the subtropical coastal waters of earlier study (Hwang & Wong, 2005) confirm that the occur- eastern Hong Kong in winter and early spring. Adults and rence of C. sinicus in coastal seas of southern China coincides juveniles first appear in the open waters of south-eastern with the north-eastern monsoon period in winter, when cold Hong Kong and Mirs Bay at the end of December. Density water masses carrying C. sinicus from the Yellow Sea and East China Sea are driven into the South China Sea through the Taiwan Strait (Hwang et al., 2004, 2006). Data presented in this study and in many previous studies suggest that temperature is an important factor regulating the distribution of C. sinicus. In the Inland Sea of Japan, the biomass of C. sinicus reaches a maximum at 208C (Uye, 2000). The upper thermal limit of C. sinicus in laboratory cul- tures is considered to be 238C (Huang & Zheng, 1986; Uye, 1988). In Xiamen Harbour, C. sinicus disappears in June when water temperature reaches 248C (Lin & Li, 1984). Zooplankton surveys conducted along the Chinese coast and the Taiwan Strait have generally confirmed that C. sinicus dis- appears when water temperature exceeds 26–278C (Wang et al., 2003; Hwang et al., 2009). In a more recent study, C. sinicus is found to be abundant in western and north- eastern Taiwan, but absent in eastern Taiwan where the main branch of the Kuroshio Current warms the surface Fig. 5. Temporal variations in the sex-ratio and the number of eggs per female water to 23.58C in January to 29.28C in July (Hsiao et al., in Calanus sinicus. Values represent average of six sampling stations. 2011). Results presented here show that temperature calanus sinicus in the coastal seas of south-eastern hong kong 139

Fig. 6. Temporal variations in the average prosome length of males, females and copepodites (CVs) at six sampling stations. Vertical bars denote the standard deviation. Broken horizontal lines mark the average prosome length at each sampling station. influences the occurrence of C. sinicus in the coastal waters of C. sinicus increases when temperature is ,208C and decreases eastern Hong Kong. There is a negative correlation between when surface water temperature begins to rise. temperature and density in C. sinicus nauplii and adults Temperature probably also influences the size of the C. (Table 1). The only increasing trend in the density of sinicus population that can be established in the subtropical C. sinicus of all developmental stages occurs between coastal seas around Hong Kong. Densities recorded in this December and early February when water temperature is study were much higher than those recorded in a study con- ,208C and decreasing. Decline in the density of all develop- ducted between 2000 and 2002 in an area near S5 and S6 mental stages begins in February when water temperature is (Hwang & Wong, 2005). While the previous study was con- just starting to increase. Very few individuals remain in the ducted in an area nearer to the shore than S5 and S6, the water column in May when water temperature approaches lowest temperature encountered was also 2–38C higher than 258C. In general, it can be concluded that the density of the 158C recorded in this study. In temperate regions, from the Bohai Sea to the East China Sea, population size of C. sinicus decreases abruptly after June, when water temperature goes up to 15–208C (Chen, 1964). In the Yellow Sea, C. sinicus has been known to survive the summer in cold waters in the thermally stratified deep areas (Wang et al., 2003; Zhang et al., 2007). The duration of the occurrence of C. sinicus also increases northward along the Chinese coast. In Hong Kong, C. sinicus occurs from

Table 1. Pearson correlation coefficients between the abundance and prosome length of Calanus sinicus of different developmental stages and various environmental parameters. Statistically significant correlations (P , 0.05, N ¼ 24) are marked by asterisks.

Chlorophyll-a Temperature Salinity

Density of eggs 0.21 –0.23 0.17 Density of nauplii –0.12 –0.43∗ 0.18 Density of copepodites –0.27 –0.27 0.29 Density of adults –0.06 –0.62∗ 0.57∗ Prosome length of copepodites –0.01 –0.69∗ 0.36 Fig. 7. Temporal variations in the prosome length of males, females and Prosome length of females –0.13 –0.36 0.19 copepodites (CVs). Values represent average of six sampling stations. Prosome length of males –0.13 –0.46∗ 0.36 Vertical bars denote the range. 140 g.-t. zhang and c.k. wong

December to May. Further north in Xiamen Harbour in the ,238C. In May, when water temperature averages .258C, Taiwan Strait, C. sinicus occurs between November and the average prosome length of the few surviving females early June (Lin & Li, 1984; Dur et al., 2007; Hsiao et al., decreases to 1.98 mm. For comparison, it is worth noting 2011). In the coastal seas of northern Taiwan, the East that the prosome length of most females in the Yellow Sea is China Sea and the Yellow Sea, C. sinicus occurs throughout .2.1 mm, although small individuals with prosome length as the year (Chen, 1992; Wang et al., 2003; Hwang & Wong, short as 1.90 mm have also been recorded (Zhang et al., 2005). 2005; Xu & Chen, 2007). The north and middle parts of the In regions with a wide temperature range, the body size of Taiwan Strait are considered to be the southern limit for the copepods may be affected by water temperature (Hirche, 1992; perennial occurrence of C. sinicus (Chen, 1964; Huang et al., Ban, 1994). The body size of calanoid copepods has been 2002). Outside Xiamen Harbour, C. sinicus disappears com- known to correlate negatively with temperature, with the pletely between June and late November from nearshore smallest individuals occurring at the highest temperature areas, but persists in deep offshore waters (Huang et al., (Halsband & Hirche, 2001; Halsband-Lenk et al., 2002). 2002). The population is able to reproduce and recruit Data presented here show a significant negative correlation rapidly when temperature becomes more favourable during between prosome length and temperature in C. sinicus CVs autumn. Shantou is located 200 km south-west of Xiamen and adult males (Table 1). Prosome length of all developmen- and 300 km north-east of Hong Kong (Figure 1). tal stages fluctuates between December and February, but Temperature in the coastal areas outside Shantou drops to follows a clear decreasing trend in March as water tempera- ,228C in November and reaches a minimum of 16.48Cin ture begins to increase. In both CVs and adults, the smallest January (Jiang et al., 2002). Calanus sinicus can be found in individuals appear in May when water temperature is high Shantou from late November to April (Huang, personal com- and the population is just about to disappear from the water munication). The time of first appearance in Shantou is almost column. similar to that in Xiamen and approximately one month Size selective predation is known to play an important role earlier than that in Hong Kong. in controlling the size composition of zooplankton commu- Correlation between salinity and density in C. sinicus nities (Brooks & Dodson, 1965). Dense schools of the larval adults is positive (Table 1). Nevertheless, the distribution of and juvenile fishes appear in Tolo Harbour in spring. C. sinicus may not be strongly influenced by salinity. In Copepods form an important part of the diet of these small Hong Kong, beginning of the rainy season and a gradual fishes (Nip et al., 2003). Planktivorous fishes rely on vision decrease in surface salinity coincide with the rise in tempera- to capture the biggest and most conspicuous prey (O’Brien, ture in spring (Figure 2). Calanus sinicus can tolerate a wide 1979). In shallow areas where C. sinicus cannot rely on vertical range of salinity (Huang & Zheng, 1986). In the Yellow Sea, migration to avoid fish predators, size selective predation may C. sinicus is well adapted to salinities .29, but its distribution favour individuals with smaller body size. is probably not limited by salinity (Wang et al., 2003; Zhang et al., 2007; Hwang et al., 2009). Surface water in the Recruitment western part of Hong Kong is strongly influenced by fresh water discharged from the Zhujiang River during the Calanus sinicus is carried into the coastal seas around Hong summer. In estuarine water with salinity ,30, C. sinicus has Kong from population centres in the Yellow Sea and the been found to occur in densities comparable to those recorded East China Sea every winter by the China Coastal Current in this study (Wong et al., 2000). (Hwang & Wong, 2005). In the eastern part of Hong Kong, No significant correlation is found between chlorophyll-a C. sinicus first appears in the open waters outside Mirs Bay concentration and the abundance of C. sinicus (Table 1). In in December and reaches the inner part of Tolo Harbour the Yellow Sea, non-phytoplankton food such as ciliates approximately one month later in January. This sequence of formed a significant portion of the diet of C. sinicus (Huo appearance suggests that C. sinicus in the northern part of et al., 2008), suggesting that the abundance of C. sinicus is the South China Sea was first recruited to the open seas not necessarily related to the supply of phytoplankton. On around Hong Kong, and then to the shallow, semi-enclosed the other hand, the egg production rate of C. sinicus in the waters of Mirs Bay and Tolo Harbour. The delay in the Inland Sea of Japan declined when chlorophyll-a concen- appearance of C. sinicus in Tolo Harbour is probably due to tration in phytoplankton .5 mmwas,0.5 mgl21 (Uye & the low level of water exchange between the semi-enclosed Murase, 1997). Chlorophyll-a concentrations at the two bay and the adjacent sea areas. The copepod community in stations in the south-eastern corner of Hong Kong dropped Tolo Harbour is marked by a scarcity of large species below 0.5 mgl21 in April and May, therefore decline in (Zhang & Wong, 2011). Calanus sinicus is a prominent chlorophyll-a concentrations may contribute to the rapid dis- member of the zooplankton community in Tolo Harbour appearance of C. sinicus after March. and Mirs Bay because of its large size. Calanus sinicus adults and copepodites that enter into the shallow waters of Tolo Prosome length Harbour are probably removed rapidly by small fishes which make their appearance in early spring (Nip et al., Seasonal changes in prosome length suggest that the body 2003). Indeed, there is a shoreward decrease in abundance size of C. sinicus may be influenced by temperature. of C. sinicus even during the period of peak densities in late Between December and early March, the average prosome January and early February, and this could be caused by pat- length of adult females varies between 2.29 and 2.34 mm, terns of the current as well as predation. and is comparable to the 2.22–2.38 mm recorded for popu- The presence of ovigerous females suggests that reproduc- lations in the Yellow Sea (Zhang et al., 2005). The average tion occurs between January and March. The maximum prosome length of females decreases to 2.14 mm in the clutch size of 111 eggs per female was comparatively higher middle of March when water temperature is rising but still than those recorded for populations in the Yellow Sea calanus sinicus in the coastal seas of south-eastern hong kong 141

(Zhang et al., 2005) and the Inland Sea of Japan (Uye, 2000). first appears, indicate that C. sinicus in the coastal waters of Since clutch size increases with prosome length (Zhang et al., Hong Kong is recruited from populations in the northern 2005), it is surprising to note that females produce their part of the species range. The shallow seas of eastern Hong biggest clutches in early March when average prosome Kong provide only marginal habitats for C. sinicus popu- length is already decreasing. On the other hand, high food lations which have been transported into subtropical oceans concentrations in February may allow females to increase on the southern edge of the species range during winter and their reproductive effort in March. At 208C, eggs take 14 early spring by water currents. hours to hatch (Zhang et al., 2005) and develop into adults in 16 days (Uye, 1988). While there is sufficient time for eggs spawned locally to develop into nauplii and copepodites, study of the effect of mud on the survival of C. sinicus egg by ACKNOWLEDGEMENTS Uye (2000) suggests that most eggs would die very quickly as they sink and come into contact with the sediment in the We are grateful to Alle Lie, Eva Yau, Vivian Li and Y.H. Yung shallow waters of Tolo Harbour and Mirs Bay. Experiments for helping with zooplankton sampling. We thank Pan Tse in the Yellow Sea revealed that high temperature is detrimen- and K.C. Cheung for technical assistance. This work was sup- tal to the hatching of C. sinicus (Zhang et al., 2007). Continual ported by the Chinese Ecosystem Research Network (CERN), decline in the density of nauplii after March also suggests that the Chinese National Ecosystem Observation and Research most eggs do not hatch into nauplii. Network (CEORN), the Science and Technology Ministry of Decrease in the prosome length of C. sinicus begins in China (973-G1999043708 and 2006CB400606), and a Direct March and the smallest females appear in May, just before Grant for Research from The Chinese University of Hong the disappearance of the species from the study area. While Kong. selective predation by visual predators may favour smaller individuals, the observed seasonal pattern in body size suggests that environmental conditions, particularly high REFERENCES temperature, in the subtropical seas around Hong Kong are detrimental to C. sinicus after March. Change in the direction Ban S. (1994) Effect of temperature and food concentration on post- of the monsoon winds, discontinuation in the intrusion of embryonic development, egg production and adult body size of cala- cold water masses from the north, and influx of warm water noid copepod Eurytemora affinis. 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